Chitosan is a basic polysaccharide, the average molecular weight of which is generally greater than 500,000, which is made up of .beta.-2-amino-2-deoxy-D-glucopyranose residues bonded in the 1,4-position. Chitosan can be obtained by deacetylation of chitin, which itself can be extracted, for example, from the carapace of crustaceans where it is present in a significant quantity.
Because of its chemical structure, chitosan is resistant to hydrolysis at the oside bonds in a basic medium, is insoluble at a pH greater than 6.5, and soluble and hydrolyzable under acid conditions. Taking into account its film-forming properties, chitosan could be an agent of choice for the protection of various substances whose release must be controlled as a function of pH. However, chitosan has the drawback of being insoluble in the customary organic solvents, which makes it difficult to produce films or coating layers.